With the latest release of Thunderbird 17, the menu bar and tabs are drawn into the title bar. On Windows XP, this then looks like the following:
In the default theme, the active title bar is blue. Thunderbird adds more stuff to the title bar which increases the blue area. This is not acceptable but fortunately this can be configured. Go to Settings > Advanced > Config Editor (in the Advanced Configuration section).
Search for the setting mail.tabs.drawInTitlebar and change the value to false (e.g., by double-clicking on it). And voilà, much better: